What are my options here to take an existing user with an 8700 on our BES and also try to get an 8100 working when they are out? If I create another user like usertmp and forward all mail from the real user to usertmp that would ensure both BB's get the same email but doesn't sync calendar, etc. I do have another exch server and BES on a different network if somehow I could sync the same user across both with one BES having the 8700 and the other having an 8100...

Any real options here cause anything I can think of has various pitfalls.

Well there will be pit falls either way because BES only supports one device per mailbox. Like you said, through mail forwarding you could at least activate a second handheld to the temp user but then the sent items wont sync...

Why do you need the user to have two handhelds at the same time? Pick one and stay with it. Or if they need to change then have them swap PINs whenever they need to.